Children don't say their first words until around the time of their first birthday, but research using experimental methods has shown that infants learn a great deal about their language before they start saying words. Researchers now believe that infants learn to perceive their language's speech sound categories (consonants and vowels) in the first year, and they learn to recognize dozens of words (even if they don't know what they mean). This project will help us determine how this learning happens, and how it helps children make further progress in language learning over the second year. The project begins by following up on the investigator's discovery that even infants as young as 6 months already know what some words mean; using recently developed techniques, it will be possible to tell whether infants learn abstract words as well as more concrete ones, and what kinds of learning mechanisms may be responsible. The project will also support study of the acoustics of infant-directed speech and testing of computational models of how infants might learn vowels, starting from a theory stating that knowing words helps infants learn speech sound categories. A second set of experiments will test how infants and toddlers learn to tell when two different-sounding words really are different words (like ten and tin) and when they are the same (like ten spoken in different linguistic environments). Thus, the project will trace the development of speech sound perception and interpretation from early infancy into the second year, aiming for mechanistic explanations of the child's first steps in language acquisition.